1. Field of the Invention
The present invention relates to a method of manufacturing compound oxide that can be used as a catalyst for purifying exhaust gas from an internal combustion engine.
2. Description of the Related Art
The compound oxide is an oxide in the form of a compound that is formed out of two or more kinds of metal oxides, and is one in which there is no oxoacid ion as a structural unit. One important application of the compound oxide is a catalyst, and a catalyst carrier. In particular, a catalyst for purifying exhaust gas from an internal combustion engine is known as the application. An example of the method of manufacturing an exhaust gas purification catalyst is described in Japanese Patent No. 3466856.
The method described in Japanese Patent No. 3466856 is a method that may be referred to as the so-called microemulsion method. According to Japanese Patent No. 3466856, a metal ion having catalytic activity is contained in aqueous phases in the water droplets (micelles or reversed micelles) contained in a water-in-oil (W/O-type) microemulsion. This microemulsion is mixed with a solution, in which a metallic compound for carrying metal that forms a hydroxide or an oxide when it is hydrolyzed is dissolved, and the mixture is stirred, so that the metal-carrying metallic compound is hydrolyzed at the interface of the reversed micelles, and primary particles of the resultant compound are produced in the reversed micelles. The primary particles aggregate in the aqueous phases in the reversed micelles to produce secondary particles, and, in addition, the reversed micelles repeat collision, fusion, and separation. Thus, aggregation of the secondary particles gradually proceeds, and the aggregates of the secondary particles (so-called tertiary particles) that have been grown to a certain size are washed, dried, and then fired to obtain metal compound oxide powder.
Japanese Patent Application Publication No. 2004-345890 (JP-A-2004-345890) describes that, when metal compound oxide is manufactured using the reversed micelle method, aggregation of secondary particles is controlled by adjusting pH, whereby the diameter of pores between the secondary particles is controlled.
The method described in the above Japanese Patent No. 3466856 is a method in which primary particles of a precursor of compound oxide are produced in micelles or reversed micelles dispersed in an oil phase, and secondary particles that are aggregates of the primary particles are synthesized. When the diameter of the micelles is appropriately controlled, secondary particles are stably synthesized in the micelles. However, because the micelles have a diameter of about several tens of nanometers, Brownian movement of the micelles is caused, and therefore, collisions between the micelles relatively frequently occur.
When the micelles collide with each other, the micelles are fused into one body with a certain probability. Thus, the aqueous phases in the micelles are coalesced with each other, and therefore, the secondary particles that have not aggregated into sufficiently large particles aggregate. This state is a state in which secondary particles having a relatively simple shape and a relatively small size aggregate, and the aggregates are substantially close to those formed of the primary particles. Thus, the possibility that the surface area and the pore volume of the obtained compound oxide become small, is high.
When the method described in JP-A-2004-345890 is used, in the case of a compound oxide containing a water-soluble component (a component the hydroxide of which is dissolved in water in a certain pH range, or at any pH), such as Ca, Sr, La, and Nd, the pore volume and the surface area are not always increased, and, in some cases, a carrier with low thermal resistance is formed. In the case of a compound oxide, such as CaZrOx, that contains a water-soluble component, for example, when it is attempted to cause precipitation at a pH lower than the isoelectric point of the compound oxide, Ca2+ ions are easily dissolved, and the dissolved Ca2+ ions adhere to the outside of the compound oxide during the subsequent aggregation operation. As a result, the outer portion is apt to become rich in calcium, and on the other hand, the inner portion is apt to become rich in zirconium. When the compound oxide in such a state is fired, homogenization occurs through a solid phase reaction, which is accompanied by the growth of particles. Accordingly, as compared to the case where the compound oxide is homogeneous from the beginning, thermal degradation in properties as a catalyst carrier occurs more rapidly.